This application relates to optical dispersion, and more specifically, to techniques for monitoring and controlling optical dispersion in optical media and transmission systems, including optical fiber links and optical wavelength-division multiplexing (WDM) fiber systems.
Many optical media exhibit various dispersion effects such as chromatic dispersion or polarization-mode dispersion (PMD). In chromatic dispersion, optical signals or spectral components with different wavelengths travel at different group velocities. Hence, the refractive index of an optical medium with chromatic dispersion changes its value with wavelength. The polarization-mode dispersion occurs in birefringent optical media which exhibit different refractive indices for light with different polarizations along two orthogonal principal directions. Therefore, an optical signal, that comprises two components along the two orthogonal principal directions for each frequency, can be distorted after propagation through the transmission medium since the two components propagate in different group velocities. This polarization-mode dispersion can cause different polarization components, even at the same wavelength, to travel at different group velocities.
Optical fibers are widely used in transmission and delivery of optical signals from one location to another in a variety of optical systems, including but not limited to fiber links and fiber networks for data communications and telecommunications. Dispersion effects, such as chromatic dispersion and polarization-mode dispersion, may cause various effects in optical fibers. For example, dispersion may distort optical pulses and hence cause noise, errors, or performance degradation in optical communication systems. More specifically, dispersion can cause the pulse broadening and thus limit the transmission bit rate, the transmission bandwidth, and other performance factors of an optical communication system. These and other effects of the optical dispersion are undesirable, especially in fiber systems to transmit bit rates higher than 10 Gbit/s over long distances such as 100 km.
It is therefore desirable to measure and monitor the optical dispersion in an optical medium in order to control or manage the effects of the optical dispersion.